The invention pertains to a viscous friction clutch according to the preamble of Claim 1 for driving a cooling fan in a motor vehicle.
Viscous friction clutches or so-called viscous clutches are known and preferably used as fan clutches for driving a cooling fan in motor vehicles. The cooling fan is driven by the internal combustion engine of the motor vehicle via the friction clutch, wherein it is known to realize a temperature-dependent control with the aid of a bimetal or electromagnetic control in dependence on various parameters such as, e.g., the coolant temperature. In viscous friction clutches, the torque in a working chamber is transmitted by shearing a shear fluid, for example, commercially available silicone oil. The silicone oil circulates between a supply chamber and the working chamber with the aid of a controlled supply device and a return device. The filling level of the working chamber defines the transmittable torque.
The invention concerns the problem of the so-called cold start performance of a viscous friction clutch, i.e., the performance of the clutch when it is engaged after prolonged inactivity of the motor vehicle internal combustion engine. In the clutch rest phase, oil drains from the supply chamber into the working chamber such that the working chamber is partially filled and causes the cooling fan to start up when the cold internal combustion engine is started. Such a start-up is undesirable when the internal combustion engine is cold, and causes undesirable noise as well as unnecessary energy consumption. This phenomenon occurs, in particular, with electrically controlled viscous friction clutches because the electromagnetic coil of the clutch is currentless when the engine is turned off so the supply device between the supply chamber and the working chamber is open. This is required for fail-safe reasons. Measures for solving this problem, which is also referred to as “morning sickness” in the relevant English patent literature, have already been proposed, e.g., in EP 1 418 361 A2: this publication proposes an accumulator or storage body with a collection chamber for an electrically controlled viscous friction clutch, wherein said accumulator or storage body is arranged radially between the supply chamber and the working chamber of the viscous friction clutch. When the clutch is at standstill, the oil flowing down from the supply chamber due to gravity is collected in the collection chamber of the storage body and prevented from entering the working chamber. This is intended to provide an improved cold start performance independent of the angular position of the accumulator.
DE 197 53 725 C2 of the applicant discloses an electrically controlled viscous friction clutch for driving a cooling fan in a motor vehicle. The known clutch features a driving disk that is driven by the internal combustion engine of the motor vehicle and is supported in a clutch housing, on which a fan wheel is mounted. An annular supply chamber arranged in the driving disk is closed toward the outside by means of an intermediate disk, and features a valve opening that can be controlled by means of a valve lever. The valve lever is actuated by an electromagnetic coil with the aid of an armature plate. In the currentless state, the valve lever is lifted off the valve opening such that oil can flow from the supply chamber into the radially outer working chamber. The oil is returned from the working chamber into the supply chamber by means of a return device. The clutch therefore features a device for supplying and for returning oil. As mentioned above, this clutch also has the disadvantage that oil can flow from the supply chamber into the working chamber while the clutch is at standstill, so that an undesirable start-up of the cooling fan can take place when the clutch is restarted.
Based on this state of the art, the present invention aims to improve the cold start performance of viscous friction clutches of the initially cited type.